Helium (He) is the second element listed on the periodic table. It is named after the Greek god of the sun, Helios. Helium was first discovered to exist on the sun during a solar eclipse before it was even found on earth[1]. Helium has an atomic number of two and has an atomic weight of roughly 4.003. In its normal state its nucleus contains two protons and two neutrons. Helium is a noble gas and is one of the must unreactive elements. It is colorless and tasteless. It is lighter than air and is regularly used to fill balloons[2].

Contents

Properties

Helium (not to scale)

Helium is a noble gas and has a full valence shell of electrons, however, it only has two electrons that fill the 1s energy level . It is colorless and tasteless. It is the second most unreactive element, behind neon. It is lighter than air and is often used in balloons. Helium has a lifting force close to one gram a liter. Helium can be a liquid or even solid, but they can only occur in those states at temperatures near absolute zero. It is less dense than air and because of this it causes your voice to sound much higher when you inhale it. Sinse helium is so light, it often rises and can completely leave the atmosphere. The rate at which helium is being lost and used is much faster than the rate at which the reserves of the element are being replenished[3].

Occurrences

Helium is the second most abundant element in the universe and in our solar system, behind hydrogen. Around 13.3 percent of all the atoms in the universe are helium atoms. Helium and hydrogen together make up 99.9 percent of all atoms in existence. Even though helium is such an abundant element, here on earth it is only the sixth most abundant element in the atmosphere. Nitrogen, oxygen, argon, carbon dioxide, and neon all are found in greater abundance. Helium itself, only composes about 0.000524 percent of the air in the atmosphere. It is not possible to produce an accurate estimate on the amount of helium that can be found in the earth's crust. The reason for this being that the helium found in the crust forms as a result from radioactive decay, but the atoms quickly escape into the atmosphere[2].

Helium theoretically, could be isolated out of the air. The air must first be liquefied, this would take very low temperatures. If the liquid gasses were then allowed to evaporate, you would be left with helium. This method is very impractical and not used. There is a much more effective method in use. The gas often occurs along with natural gas in reservoirs deep under the Earth's crust. When wells are dug to collect the natural gas, helium makes its way to the surface along side the natural gas. Helium can then be separated from natural gas very easily. The temperature of the mixture is lowered, causing the natural gas to liquefy. Once the liquid natural gas is removed, gaseous helium remains[2].

Another way helium is produced on earth is from radioactive decay. All elements that have 84 or more protons are unstable. These atoms will eventually under go some sort of decay. One type of decay produces a helium nucleus. This is called alpha decay. During alpha decay, and unstable atom will lose two neutrons and two protons(an alpha particle). This particle that is emitted is the nucleus of a helium atom. When it is first emitted, it has no electrons so it has a charge of plus two. However, the positively charged helium atom will quickly pick up two electrons in order to neutralize itself[4].

Uses

Helium is a very valuable resource and ahs several popular uses. One of helium's main uses is in the production of lighter-than-air crafts. Theses aircrafts and other balloon type objects are able to float because of helium's buoyance as well as its noncombustibility. Because of helium's valence shell is full, it is very unreactive. This unreactive gas can be used as an atmosphere in refining titanium and zirconiummetals, in growing crystals of silicon and germanium for semiconductors, and in electric arc welding. A mixture of helium and oxygen can be supplied as a breathing mixture for deep-sea divers and caisson workers. It is also used in decompression chambers; its use reduces the risk of caisson disease because helium is less soluble in human blood than nitrogen. Helium is also used to pressurize the fuel tanks of liquid-fueled rockets. Liquid helium is essential for many low temperature applications[5].

Discovery and History

Helium line spectra.

Helium is the second most occurring element in the universe. Before it was even known to exist on the earth, it was discovered on the sun. Pierre Jules César Janssen was the first man to note the existence of helium[6]. As he was observing a solar eclipse in India during the year 1868, he noticed the sun was giving of a yellow line spectra. Other astronomers noted this fact as well. One such astronomer was the Englishman Norman Lockyer. Lockyer is credited with naming the element Helium, after the Greek god of the sun, Helios. Helium can be viewed at 587.49 nanometres in the spectrum of the Sun's chromosphere[1].

At first, it was thought that helium could only exist on or in the sun. Scientists came to this conclusion after they failed to produce the same spectral results in a lab. However this setback did nothing to slow down the search for the element on earth[1]. The hunt for helium concluded in 1895. Sir William Ramsay, a Scottish chemist, was experimenting with a mineral containing uranium called clevite. He subjected the mineral to acids, and then collected the gasses that were produced[6]. Sir William was looking and expecting to find argon. However, after separating the nitrogen and oxygen from the gas he liberated with sulfuric acid, he noticed a bright-yellow line that matched the spectral line observed in the sun. Ramsey then shipped samples of the gas to Sir William Crookes and Sir Norman Lockyer who proceeded to verify that it was helium. That same year, two Swedish chemists Per Teodor Cleve and Abraham Langlet, identified and isolated helium in cleveite in Uppsala, Sweden. They were also able to calculate helium's atomic weight[1].